One key feature in the Wideband Code Division Multiple Access (WCDMA) standard is the ability to simultaneously establish links to multiple cells. This is feature is commonly referred to as soft handover when the cells are located at different sites, softer handover when the cells are located at the same site or macro diversity etc. The set of cells that a mobile station which may be termed User Equipment (UE) is connected to is commonly denoted the active set.
The WCDMA active set is formed based on measurement reports from the User Equipments (UEs). These measurements are performed on a reference signal for each cell. The reference signal is a signal used for measurements and which may also be used for synchronization. In the case of WCDMA, the Common Pilot Channel (CPICH) is used. For all relevant cells, the channel quality/channel condition, such as the received quality or strength is determined. IN the case of WCDMA the measured Ec/No (chip power relative total received signal power) is compared for all relevant cells. Also, the path gain or CPICH level (CPICH Received Signal Code Power, RSCP) may additionally be determined. When determining the active set.
Assuming that a UE is connected to a first cell, cell A, and that the UE is gradually approaching a second cell, cell B, the quality of the measured reference signal, such as the CPICH Ec/No, of cell B will at some stage be within a specified window relative the best serving cell, cell A. This triggers an event and the UE sends a measurement report to the serving Radio Network Controller (RNC) that cell B is within range. In response to such a measurement report Cell B is added to the active set of this particular UE via an Active Set Update message transmitted from the RNC to the UE. Also, the cell B has been informed by the RNC that a link to the UE has to be established.
In FIG. 1 such a scenario is schematically depicted. Thus, in FIG. 1, the two links to the first and second cells, i.e. cell A and Cell B have equal path gain g0. Further it should be noted that the active set is determined based on the signal quality of the measured reference downlink signal. The following events may typically used to determine the active set:
A. A new cell has entered reporting range (used when there are room for yet another cell in active set)
B. A cell in active set has leaved reporting range
C. A new cell is better than a specified active set cell (used when active set is full)
D. A different cell is better than previously best cell in active set.
If the uplink and downlink path gains of the links to/from the two cells are the same, then the active set, despite only evaluated for the downlink, constitute a good set of cells also for the uplink However, if the uplinks and downlinks are different, and furthermore the downlinks from the two cells are different, the active set might be relevant for the downlink, but much less relevant for the uplink.
One such example is when the uplinks and downlinks are unbalanced, for example when low noise amplifiers are used in the uplink to compensate for feeder losses between the Radio Base Station RBS and the antenna. The RBS amplifier feeds the antenna via a feeder cable, which has a certain feeder cable loss. This can be significant, for example in some cases it may be as high as 10-12 dB. In the uplink, the signal is sent from the antenna down to the RBS via a feeder cable causing a similar loss. This could give a very weak signal at the RBS. To compensate for this loss the received signal is amplified by a low noise amplifier directly connected to the antenna. This amplifier is sometimes also referred to as Tower-mounted amplifier (TMA). An exemplary RBS and antenna configuration is illustrated in FIG. 2. In FIG. 2, the antenna 201 is connected to a TMA 203. The TMA receives signals from the Antenna 201 on a TMA Antenna port 207. The TMA 203 is further connected to a Radio Base Station (RBS) 205. The RBS 205 receives signals from the TMA via RBS port 207.
With a significant feeder loss in cell A, and a negligible loss in cell B, the point where the equal CPICH Ec/No from the two cells is shifted, and the perceived combined downlink feeder loss and path gain is lower at this point.
In FIG. 3 an illustrative example is shown. In the example shown in FIG. 3, the path gain is (g0+3 dB)−6 dB from cell A including both path gain and feeder loss, and g0−3 dB from cell B. However, for the uplink, the gain from this downlink mid point is g0+3 dB to cell A, but only (g0−3 dB) to cell B. At this downlink mid point, there is probably very little use of cell B. This means that when moving from cell A to cell B, cell A will at some stage be removed from the active set since it becomes worse downlink-wise. However, at this stage, the uplink to cell A can still be relevant, and worth keeping.
The active set will in some cases be less relevant for the uplink There are however situations when it is more important to have a relevant active set uplink-wise. One such example is High Speed Packet Access (HSPA), where the High Speed—Downlink Shared Channel (HS-DSCH) is transmitted only from the best cell in the active set, i.e. the serving cell. The serving cell is typically determined to be the cell of the active set providing the best signal quality. On the other hand, the Enhanced Dedicated Channel (E-DCH) is received by all base stations, Node Bs, in the active set, the uplink data rate in a practical case might be limited by the non-serving cells in the active set, i.e. the cells of the active cell other than the serving cell.
According to the above, the HS-DSCH is hence only transmitted from the serving cell to the UE and it is therefore important that the cell providing the best down link conditions is the serving cell. Similarly, to obtain good conditions for the E-DCH it is important that the cell providing the best conditions in the up-link is among the cells forming the active set.
Hence, there exist a need for a method and a system that is able to provide an active set that is better suited for evolving radio communication systems and which is able to provide a relevant active set for the up-link of a cellular radio communication connection.